Specific heat measurements of the lattice contribution and spin-density-wave transition in (TMTSF)2X (X = PF6and AsF6) and (TMTTF)2Br salts

The specific heats of the quasi-one-dimensional organic compounds (TMTSF) 2 X (X = PF 6 , AsF 6 and ClO 4 ) and (TMTTF) 2 Br were measured over the temperature range from 1.8 to 40 K. The lattice contribution follows a T 3 -law only in a small temperature range below 3 K, and shows a typical bump in C / T 3 at about 7.0 K for every compound (4 K for (TMTSF) 2 ClO 4 ). We interpret this behaviour as a phonon dimensionality crossover from a low- T tridimensional behaviour, towards a lower-dimensionality T -behaviour (with = 2.3 to 2.5) above 3 K, plus two additional Einstein modes in agreement with the frequencies of soft lattice modes detected by far-infrared spectroscopy. The spin-density-wave (SDW) transition temperature was found to take the values 12.2, 12.4 and 11.7 K for the compounds (TMTSF) 2 PF 6 , (TMTSF) 2 AsF 6 and (TMTTF) 2 Br, respectively. The magnitude of the SDW transition anomaly is 4.5%, 2.5% and less than 1% of the total specific heat for each sample, respectively. The magnitude of the specific heat anomaly for (TMTSF) 2 PF 6 is too large to be explained by the electron spin contribution alone, which implies that the lattice is also involved in the transition. From our accurate measurements, we confirm that the SDW transition is a second-order transition without any characteristics of first order, like hysteresis and latent heat. The specific heat was found to be time dependent over the whole measurement temperature range as its value is quite sensitive to the kinetics process for all three compounds.